A fundamental goal of biomedical research is to understand the processes that regulate gene expression during development and disease. The mouse alpha-fetoprotein ATP is particularly well suited for this. AFP is expressed at high levels in the fetal liver but is off in the adult liver. This is due to a 10,000 reduction in transcription during the perinatal period. The AFP gene can be reactivated in the adult liver in response to injury and in hepatocellular carcinomas. These aspects of AFP regulation-AFP activation during hepatogenesis, repression at birth, and reactivation in liver cancer and regeneration-have led to considerable interest in the control of this gene. Postnatal AFP repression is regulated in part, by a locus called Alpha-fetoprotein regulator 1 (Afr1). Afr1 was originally identified by differences in AFP levels in different mouse strains. Thus, unlike a majority of mammalian factors controlling gene expression that have been identified using biochemical approaches, Afr1 was revealed genetically. Of particular interest, Afr1 appears to regulate AFP by a mechanism that couples transcription to post-transcription events. While a connection between these events has been established in the literature, mechanisms that exist to modulate these connections are only beginning to be uncovered. Using tools of contemporary molecular genetics, we propose to identify the Afr1 gene by positional cloning. We can then understand how Afr1 regulates AFP and we are likely to learn more about the transcription/post/transcriptional connections as well as how this may be developmentally regulated to control gene expression.